Using GPU outside of rendering in background mode
The Python you are writing is - even within Blender's interpreter - just Python: an interpreted language running on the CPU. There are ways to leverage the GPU in Python (eg. PyCUDA), but I would be very surprised that using the GPU would help here. As long as you are not part of Blender's own codebase (which entails recompiling the beast and diving into C++ or something that can be linked to it), you have to go through the Python API and whatever advantage there is to generating the planes on the GPU will be eclipsed by handing them over to Blender one by one though the interpreter.